Generating high speed ultrasonic thick slice imaging by combining data in elevation direction via volumetric rendering process

ABSTRACT

An ultrasonic diagnostic imaging system scans a plurality of planar slices in a volumetric region which are parallel to each other. Following detection of the image data of the slices the slice data is combined by projecting the data in the elevation dimension to produce a “thick slice” image. Combining may be by means of an averaging or maximum intensity detection or weighting process or by raycasting in the elevation dimension in a volumetric rendering process. Thick slice images are displayed at a high frame rate of display by combining a newly acquired slice with slices previously acquired from different elevational planes which were used in a previous combination. A new thick slice image may be produced each time at least one of the slice images is updated by a newly acquired slice. Frame rate is further improved by multiline acquisition of the slices.

The present application is a continuation of U.S. patent applicationSer. No. 12/594,885 filed Oct. 6, 2009, which is the U.S. National Phaseapplication under 35 U.S.C. § 371 of International Application No.PCT/IB2008/051346, filed Apr. 9, 2008, which claims the benefit of U.S.Provisional Application Ser. No. 60/911,580 filed Apr. 13, 2007. Theseapplications are hereby incorporated by reference herein.

This invention relates to medical diagnostic ultrasound systems and, inparticular, to ultrasound systems which acquire and display an imagefrom image data in the elevational dimension at high frame rates ofdisplay.

Ultrasonic diagnostic imaging is an imaging modality which forms imagesof coherent signal information. The nature of the coherent ultrasonicsignals used, like the monochromatic lightwaves used for holographicimaging, results in constructive and destructive interference of thewaves in the medium being imaged. As a result, the image contains noisein the form of a random mottling of the image known as “speckle.” Sincethe speckle pattern of an image is constant and does not vary with time,the common approach to reducing the effect is to combine uncorrelatedimage data and reduce the speckle by an averaging effect proportional tothe square root of two. The types of uncorrelated data used aretypically data that are of different frequencies or acquired fromdifferent look directions, commonly known as frequency compounding (see,e.g., U.S. Pat. No. 4,350,917 to Lizzi et al.) and spatial compounding(see, e.g., U.S. Pat. No. 4,649,927 to Fehr et al.)

U.S. Pat. No. 6,464,638 to Adams et al. describes a new approach tospatial compounding which makes good utilization of probes designed forthree dimensional imaging. In the Adams et al. technique a 3D imagingprobe acquires images of planes which are substantially parallel to eachother in the elevational dimension, the dimension normal to the imageplane. In a typical implementation, Adams et al. use a probe withelectronic beam steering and focusing in both azimuth and elevation toacquire not only an image of the slice plane of interest but also imageplanes offset from that slice plane. The slices are then combinedelevationally and the at least minimally uncorrelated data in theelevational dimension effects speckle reduction by spatial compoundingin the elevational dimension.

While the implementation described in Adams et al. utilizes parallelprocessing to acquire multiple scanlines from a single transmitinterval, it is nonetheless necessary to acquire a full data set of theelevational slices before a spatially compounded image can be formed.This is longer than the time required to acquire a single uncompoundedslice image and hence the frame rate of display of the real timesequence will be slower than the frame rate of uncompounded real timeimaging. Accordingly it is desirable to be able to produce spatiallycompounded images at higher real time frame rates of display.

In accordance with the principles of the present invention, a diagnosticultrasound system and method are described which produces spatiallycompounded images from data in the elevational dimension at high framerates of display. A plurality of slices are scanned with a 3D imagingprobe which are substantially parallel in the elevational dimension.Following the acquisition of a new slice, the image data of the newslice is combined in the elevation dimension with the image data ofpreviously acquired slices, then displayed. Various techniques can beused to combine the elevation data such as averaging, weighting ormaximum intensity projection. A new spatially compounded image frame isproduced for display in less time than is required to acquire the totalnumber of slices being combined. In an illustrated implementation theframe rate of display is further increased by multiline acquisition ofscanlines from several elevational slices at the same time.

In the drawings:

FIG. 1 illustrates a plurality of sector slices acquired in theelevational direction.

FIG. 2 illustrates a plurality of rectilinear slices acquired in theelevational direction.

FIG. 3 illustrates a plurality of slices which are at different angularincrements in the elevational direction.

FIGS. 4a-4c illustrate the acquisition of multiple slices simultaneouslyby multiline acquisition in accordance with the principles of thepresent invention.

FIG. 5 illustrates in block diagram form an ultrasonic diagnosticimaging system constructed in accordance with the principles of thepresent invention.

FIG. 6 illustrates in block diagram form a second implementation of anultrasonic diagnostic imaging system constructed in accordance with theprinciples of the present invention.

FIG. 7a illustrates a dual ported memory used for slice storage in animplementation of the present invention.

FIG. 7b illustrates partitioning of memory areas in an implementation ofthe present invention.

Referring first to FIG. 1, a volumetric region 10 is shown inperspective. In this example the volumetric region 10 is sector-shapedand contains a plurality of planar sector-shaped areas which arereferred to herein as “slices.” Four slices 12-18 are illustrated inthis example. The slices are oriented parallel to each other in theelevation direction with their azimuth and elevation dimensionsindicated to the right of the drawing. Each slice may be scanned by anarray transducer located above the volumetric region by transmittingsuccessive scanlines across a slice 12-18 in the azimuth direction andprogressing from slice to slice in the elevation direction.

FIG. 2 illustrates a rectilinear volumetric region 20 which alsoincludes a plurality of slices oriented in parallel in the elevationdirection. Four such slices 22-28 are shown in the drawing. These slicesmay be scanned in the same manner as the slices of FIG. 1 by atransducer array located above the volumetric region 20. In this examplethe slices are scanned by parallel scanlines in the azimuth directionrather than by angularly incremented scanlines from a common origin asis the case in the example of FIG. 1.

FIG. 3 provides another example of slices of a volumetric region. Theseslices are of a pyramidal volumetric region with an apex 34 at the topof the volume. In this example four sector-shaped slices S₁-S₄ are shownin an “edge-on” view. That is, the elevation direction of the slices isindicated by the arrow 32, and the azimuth direction is into the planeof the drawing. The azimuth and elevation directions with respect to thearray transducer 30 are shown above the transducer array. In thisexample neighboring elevation slices are substantially parallel and areseparated from each other by an angular increment Δφ.

In each of these examples a single slice of a volume may be scanned anddisplayed. But in accordance with the principles of the presentinvention, a plurality of slices which are elevationally aligned arescanned and their data combined to form an image for display. Since eachof the elevationally distinct slices is scanned by scanlines havingdifferent transmit-receive signal paths, each of the slices will exhibitits own unique speckle pattern. By combining the image data of aplurality of slices which define a thickness in the elevation dimension,the speckle artifact of the combined image will be reduced.

In accordance with a further aspect of the present invention, the slicesmay be scanned at a high speed by multiline acquisition. In multilineacquisition, one transmit beam insonifies multiple receive linelocations and multiple receive lines are acquired in response to thesingle transmit event. FIGS. 4a-4c provide an example of multilineacquisition of four slices S₁-S₄ which are arranged in parallel in theelevational dimension. Each slice is made up of receive lines arrayed inthe azimuth direction and identified in the drawing as L1, L2, . . . Ln,where “n” may be 128, for instance. In the view of FIG. 4, each receiveline is being viewed axially as it would from the perspective of thetransducer array. Rather than transmit a single transmit beam down eachline and receive echoes from only that receive line, four receive linesare insonified by a single transmit beam. In the example of FIG. 4a atransmit beam TxA1, outlined radially, insonifies receive lines L1 andL2 of slice S₁ and receive lines L1 and L2 of slice S₂. Thus, tworeceive lines in azimuth and two receive lines in elevation, a total offour receive lines, are acquired simultaneously and processed. See,e.g., U.S. Pat. No. 5,318,033 (Savord) for an explanation of theprocessing of simultaneously received multilines. FIG. 4b illustratesthe next transmit event, in which a transmit beam TxA2 insonifiesanother four receive lines, L3 and L4 of slice S₁ and receive lines L3and L4 of slice S₂. Scanning proceeds in this manner until all of thelines of slices S₁ and S₂ have been acquired. Thus in the intervalduring which the full azimuth of a slice has been scanned, from line L1through line Ln, echo data from two slices, S₁ and S₂, has beenacquired. The process then continues with a second azimuth scanninginterval as shown in FIG. 4c with the scanning of receive lines L1 andL2 of slice S₃ together with receive lines L1 and L2 of slice S₄ bytransmit beam TxB1. Slices S₃ and S₄ are scanned during this secondazimuth scanning interval in the same manner as slices S₁ and S₂ wereacquired during the first. In these two scanning intervals all fourslices S₁-S₄ are scanned in the time required to scan a single slice inthe conventional line-by-line approach. The speed of acquisition andhence the frame rate of display have been increased by a factor of fourby the use of this 4× multiline acquisition.

An ultrasound system constructed in accordance with the principles ofthe present invention is shown in block diagram form in FIG. 5. A twodimensional array transducer 30 is provided which electronically steersand focuses beams over a volumetric region 10 under control of amicrobeamformer 36, main beamformer 38, and beamformer controller 42.Alternatively, a one dimensional array transducer can be mechanicallyoscillated to scan the volumetric region. In this case themicrobeamformer 36 located in the probe case with the 2D transducerarray 30 controls the scanning of groups of elements called subarrays orpatches in scanning a volumetric region 10. Partially beamformed signalsfrom the microbeamformer 36 are formed into fully beamformed signals bythe main beamformer 38. A beamformer controller 42 provides controlsignals for the beamformer and microbeamformer. Further details onmicrobeamformer-controlled scanning of volumetric regions may be foundin U.S. Pat. No. 6,623,432 (Powers et al.), and U.S. Pat. No. 6,709,394(Frisa et al.), PCT publication WO 2005/099579 (Rafter) and U.S. patentapplication 60/777,831 (Savord), filed Mar. 1, 2006. In this example auser control panel 40 is coupled to the beamformer controller 42 and isoperated to control a number of parameters of the scanning of slices12-16 of the volumetric region 10, including the number of slices to bescanned, the spacing between slices, the number of transmit slices, andthe number of receive slices per transmit slice. Referring back to FIGS.4a-4c , in that example the number of slices to be scanned was four, thespacing between slices was a specified angular or linear parameter, thenumber of transmit slices was two, and the number of receive slices pertransmit slice was two.

The beamformed echo signals received from the scanned slices aredetected by a log detector 52 for B mode imaging. Alternatively or inaddition, the received echo signals may be Doppler processed by aDoppler processor 54 for the display of flow or motion in the imagefield. The B mode image data and the Doppler image data (e.g., Dopplerpower and/or velocity) of each slice are stored in slice storage buffer60. Addressing of the buffer 60 to write data into the buffer or readdata out of the buffer is controlled by memory controller 62. In animplementation of the present invention a plurality of elevationallydifferent slices are read out of the slice storage buffer 60 andcombined by a combiner 64.

The combiner 64 may combine the image data of multiple elevationallydifferent slices in various ways. Combining is preferably performed onimage data from different slices which have the same azimuth and depthcoordinates in each slice. Alternatively, raylines can be mathematicallyprojected through the multiple slices in the manner of raycasting forvolume rendering. Preferably the raylines are projected normal to theplanes of the slices. The image data intersected by each rayline is thedata which is combined. In the combining process the image data can beaveraged or can be summed and normalized. A mean or median value of thedata values can be computed, or a peak value of the data being combinedcan be used. The data from the central slice can be weighted moregreatly than the data of neighboring slices, with slice data beingweighted in relation to its distance from the central slice. Slice datacan be weighted in relation to its proximity to the viewer with slicedata in the front of the volume being weighted more greatly than slicedata in the back. The combined data thus forms a “thick slice” which canbe displayed as a planar display of a slice with characteristics ofmultiple elevationally offset individual slices. The thick slice data iscoupled to an image processor 70 for further processing such as scanconversion into the desired display format (e.g., sector or linear) andis processed into video signals by a video processor 72 for display on adisplay 76. The image data can also be saved or stored in a Cineloop®memory 78, harddrive or other image storage device. The thick slicedisplay will exhibit reduced speckle artifacts as compared to anindividual one of the acquired slices.

In accordance with a further aspect of the present invention a highframe rate of display for thick slice images may be obtained by means ofthe apparatus and techniques depicted in FIGS. 7a and 7b . FIG. 7aillustrates the slice storage buffer 60 implemented as a dual portmemory 160 which can be written to and read from simultaneously. The useof such a R/W memory 160 enables the new data of a slice being scannedby the transducer array and beamformer to be written into one area ofthe R/W memory while the data of other slices previously stored in thememory is read out and combined to form a thick slice image. The writingof new slice image data into the memory 160 is controlled by a writeaddress controller 162 a while the reading of slice image data fromother locations in the memory is under the control of a read addresscontroller 162 b. In this technique a new thick slice image can becombined for display while the image data from a new slice is beingacquired. One example of the allocation of memory for a combinedfour-slice thick slice image is illustrated by FIG. 7b . The storagearea 260 of the memory is shown to contain seven image storage areaslabeled A through G.

An example employing the 4× multiline scanning technique of FIGS. 4a-4cfor four component slices S₁-S₄ is as follows. Using the user interface40, the ultrasound system is set to scan four slices with a given slicespacing, using two transmit slices and two receive slices per transmitslice. Scanning of the first two slices proceeds during a first scanninginterval as shown in FIGS. 4a and 4b and the data of the two acquiredslices S₁ and S₂ is written into memory areas A and B. Slices S₃ and S₄are then scanned during a second interval and the data of these twoslices is written into memory areas C and D. The transducer array andbeamformer then begin to scan slices S₁ and S₂ again and write the datafrom the rescanning of slices S₁ and S₂ into memory areas E and F. Whilethese slices are rescanned, the image data of memory areas A, B, C, andD is read out of the memory and coupled to the combiner 64 where theindividual slice data is combined into a thick slice image. Theresultant thick slice image is written into memory area G, from which itis read out and coupled to the image processor (and other components asdescribed below) as needed for processing and display. In a typicalimplementation the time required to composite the thick slice image andprocess the image for display will take less time than the time requiredto rescan slices S₁ and S₂. After the rescanning of slices S₁ and S₂ iscomplete, the image data of slices S₁, S₂, S₃, and S₄ which is stored inmemory areas C, D, E, and F is read out for combining into a new thickslice image for display, and the new thick slice image is written intomemory area G to update the real time thick slice image. Simultaneously,slices S₃ and S₄ are rescanned and their slice data is written intomemory areas A and B. In the next scanning interval iteration slices S₁and S₂ are scanned again and their data written into memory areas C andD while the slice data of memory areas E, F, A, and B is combined toform another thick slice image to update the image in memory area G.This use of 4× multiline for slice acquisition and the combination ofnew slice data with the most recent data of the other slices of thethick slice image is seen to enable a frame rate of display of the thickslice image which is equal to that of a single slice scanned anddisplayed by conventional single line scanning. Thus, there would be nodegradation of frame rate when changing from conventional single sliceimaging to thick slice imaging of four component slices by thistechnique.

An implementation of the present invention has been found to beespecially useful in colorflow imaging, particular for the detection ofsmall, localized and intermittent flow conditions such as a heart valvejet. Colorflow has long been used in the detection of flow jets fromvalve leakage, a clinical application for which sensitivity faroutweighs precise image resolution. Normally this procedure takes a longtime as the clinician slowly moves the image plane around the heartvalve, looking for a short burst of color characteristic of a jet.However, with the system of FIG. 5, this procedure is considerablyenhanced. Since the combiner combines a number of elevationally distinctplanes spread over a small volumetric region in elevation, the jet neednot occur in the center plane in order to be detected. The occurrence ofa jet in the plane of an adjacent slice which is collapsed into thethick slice will enable the jet to be detected even when it is notpresent in the central slice plane of the thick slice. Furthermore, thejet is more easily detected by the reduction of speckle artifact andcolor dropout in the thick slice image. While the processing of one ofthe component slices by the Doppler processor 54 may result in blackholes in the colorflow image where destructive interference from thespeckle pattern has manifested itself, the differing speckle pattern ofthe neighboring slice may not exhibit this problem at the same point inthe image. Thus, when the colorflow slices are combined in the elevationdimension into the thick slice image, the black hole of one slice may befilled in by valid colorflow of a neighboring slice. The colorflow fieldwill appear smoother and more sensitive to out-of-central plane jetswith less far field degradation. Sensitivity of the procedure to jetdetection is accordingly enhanced.

For the production of a Doppler thick slice image, ensembles of echosignals are received from locations where flow or motion is present andare processed by the Doppler processor 54 to produce a Doppler estimateat those locations. The Doppler estimate may be one of Doppler power atthe location, or velocity or variance. Corresponding B mode images mayalso be acquired if desired so that the Doppler information may beoverlaid on structural detail framing the motion. The Doppler sliceimages are stored in slice storage 60, then combined by combiner 64using a selected combining technique. Defects in the flow or motiondisplay due to speckle or dropout are thereby reduced, and flow ormotion defects in adjacent slice planes are more easily identified bythe projection of multiple Doppler slices in the elevation dimension.Furthermore, since the acquisition of multiple temporally differentsamples from each flow or motion location will decrease the frame rateof acquisition in the Doppler mode, at least some of this frame ratedegradation may be overcome by use of the high speed thick slice displaytechnique discussed in conjunction with FIGS. 7a and 7b above.

In accordance with a further aspect of the present invention, the thickslice images are also coupled to an automated or semi-automated borderdetector (ABD) 80. As is well known, border detectors are used toidentify tissue borders in ultrasound images. The border detectors canoperate with initial user involvement to identify points on one borderin one image, then use that input to automatically identify the fullborder and the border in other images of a real time image sequence.Other border detectors operate automatically by identifying tissuelandmarks in an image then drawing borders using those landmarks. See,for example, U.S. Pat. No. 6,491,636 (Chenal et al.) and U.S. Pat. No.6,447,453 (Roundhill et al.) and US patent publication 2005/0075567(Skyba et al.) The border detector 80 identifies a tissue border in athick slice image with or without user assistance (semi-automated orautomated) and couples data identifying the location of the border inone or more thick slice images to a graphics processor 74. The graphicsprocessor 74 creates a graphic outline of the border to the imageprocessor 70 which overlays the identified border over the correspondingthick slice image. It has been found that automated or semi-automatedborder detection performs better on thick slice images than oncomparable single slice images. This is because a tissue border definedby thin tissue which is not a strong reflector of ultrasonic echoes suchas the endocardial border of the myocardium can produce a poorly definedtissue border in a single slice image. Image dropout at the borderregion can produce an ill-defined image border which is difficult totrace reliably by an automated or semi-automated process. In addition,the poorly-defined border can be further disrupted by the image specklepattern. The combining of elevationally distinct images into a thickslice image can reduce the speckle artifact and make the border moredistinct in the image. In addition, missing border segments in one slicecan be augmented by identifiable border segments in adjoining slices,causing the consolidated tissue border of the thick slice image to bemore clearly defined and hence more reliably processed and identified bythe border detector 80.

In accordance with a further aspect of the present invention, thickslice imaging is used in the diagnosis and quantification of perfusiondefects with the aid of ultrasonic contrast agents. When a contrastagent is present in a blood pool such as a blood vessel or chamber ofthe heart, the contrast agent will generally be present in considerablevolume and density in the blood pool. The relatively high concentrationof the microbubbles of the contrast agent enable quick and reliabledetection of its presence in an ultrasound image. However in perfusionstudies such as those conducted with contrast agents to detect poorlyperfused tissue such as myocardial tissue which has been infarcted, thecontrast agent is only present in small amounts in the tiny capillarieswhich perfuse the tissue. This low concentration of the microbubblesoften makes their detection and quantification difficult or unreliable.This is at a time when high resolution is required since perfusiondefects often show up as thin subendocardial regions of slower fillingas well as potentially lower blood volume. In addition, perfusionstudies are generally conducted at low transmit power levels to avoidbreaking or disrupting the microbubbles in the capillary bed and causingthem to disappear. Consequently the signal-to-noise ratio of theperfusion images is relatively low, frequently by as much as 20 dB lowerthan standard imaging techniques, causing further degradation inresolution. The resultant images can have a display dynamic range whichis 20 dB or more lower than conventional images without contrast,causing the speckle artifact to have a more pronounced adverse impact onimage resolution and the detection of subendocardial regions of poorperfusion.

Accordingly, contrast images for perfusion diagnosis and/orquantification are improved in accordance with the present invention byscanning multiple planes in the elevation dimension and projecting thesemultiple elevation slices in the elevation dimension. By performing suchoperations it is possible to reduce speckle without sacrificingresolution and signal to noise. The methods for compositing or combiningslices which have been described above may be employed, including simpleaveraging and maximum intensity projection, or using compositingtechniques from volume rendering (e.g., raycasting). By performing thesetechniques, the contrast agent speckle will be greatly reduced,subendocardial defects will be more evident, and quantificationtechniques such as parametric imaging will yield better results.Furthermore, since “destruction-replenishment” techniques requireexactly the same elevation slice to be maintained for 10 seconds ormore, thick-slice imaging will be more robust in the presence of smallmovements of the probe, since a plurality of adjacent slices are used toform the thick slice image plane. Thus, slight movement of the probe todifferent slice locations will have only minimal effect on the resultsobtained.

An ultrasound system constructed in accordance with the principles ofthe present invention for perfusion studies is shown in block diagramform in FIG. 6, in which elements previously described in conjunctionwith FIG. 5 are identified by the same reference numerals. In thissystem thick slice images of microbubble-perfused tissue which areproduced by the combiner 64 may be processed as B mode images by theimage processor 70, the video processor 72, and the display 76 for thedisplay of real time grayscale images of perfusion which exhibit betterresolution of tissue perfusion by virtue of reduced speckle caused bythe elevational slice combining process. In this example the thick slicecontrast images are also coupled to a perfusion detector 90. Theperfusion detector 90 may be constructed in the same manner as thecontrast signal detector described in PCT publications WO 2005/044108(Rafter) and WO 2005/099579 (Rafter) to detect and enhance the displayof contrast agent perfusion in the images. Alternatively or in additionthe perfusion detector may be configured as the contrast signal detectordescribed in U.S. Pat. No. 6,692,438 (Skyba et al.) to produce a coloroverlay of the B mode image which depicts perfused tissue in aqualitative color display, or a quantitative display of a perfusioncurve or curve parameter for different points in the image.

Other conceptually different approaches may be used to arrive at a thickslice image. For example, a volume of data equal to or larger than thethick slice volume may be acquired. The elevationally distinct slicesare then defined by a process known as multiplanar reformatting, bywhich the slices are identified in the data set. The slice data is thencombined in the elevation dimension to produce the thick slice image.

Other variation of the present invention will readily occur to thoseskilled in the art. For example, the concepts of the present inventionmay be employed in an implementation which does not use multilineacquisition but acquires one receive line for every transmittedscanline. Various sequence of line acquisition may be employed otherthan successive acquisition of adjacent lines such as those shown inU.S. Pat. No. 5,438,994 (Starosta et al.) and U.S. Pat. No. 5,617,863(Roundhill et al.) Higher order multiline may be employed than theillustrated 4× multiline, including a multiline order which acquires allof the component slices in one azimuthal scan sequence. Doppler modesother than colorflow may use the present invention including spectralDoppler, flow variance, and color M mode. M mode may use animplementation of the present invention which acquires and combinesspatially distinct M lines into one display M line. The techniques ofthe present invention are applicable to both fundamental and harmonicimaging.

What is claimed is:
 1. An ultrasonic imaging system, comprising: anarray transducer; a beamformer that is coupled to the array transducerand configured to control the array transducer to transmit beams and toreceive echoes from a plurality of receive scanline locations in avolumetric region; a memory that is coupled to the beamformer andconfigured to store a first set of image data comprising a plurality ofparallel, elevationally distinct slices of the volumetric region; acombiner that is coupled to the memory and configured to combine asecond image data with a portion of the first set of image data byvolume rendering in the elevation direction to produce a thick sliceimage comprising the second image data and the portion of the first setof image data; and a display coupled to the combiner and configured todisplay the thick slice image at a frame rate that is faster than thatof a time to acquire the first set of image data and the second imagedata.
 2. The ultrasonic imaging system of claim 1, further comprising abeamformer controller coupled to the beamformer that is configured tocontrol a number of the plurality of parallel, elevationally distinctslices to be scanned for the thick slice image.
 3. The ultrasonicimaging system of claim 2, wherein the array transducer is configured toelectronically focus and steer scanlines in response to the beamformer.4. The ultrasonic imaging system of claim 3, wherein the beamformercomprises a multiline beamformer.
 5. The ultrasonic imaging system ofclaim 4, wherein the beamformer controller further controls a number ofthe plurality of receive scanline locations per parallel, elevationallydistinct slice.
 6. The ultrasonic imaging system of claim 1, wherein thememory comprises a first memory area configured to store the first setof image data and a second memory area configured to store the secondimage data.
 7. The ultrasonic imaging system of claim 6, wherein thememory comprises a third memory area configured to store the secondimage data and the portion of the first set of image data.
 8. Theultrasonic imaging system of claim 1, wherein the combiner is configuredto perform at least one of summing the first set of image data in theelevation direction, averaging the first set of image data in theelevation direction, weighting the first set of image data in theelevation direction, or detecting a maximum value of the first set ofimage data in the elevation direction.
 9. The ultrasonic imaging systemof claim 1, comprising a B mode processor or a Doppler processorconfigured to process the first image data to produce a B mode image ora Doppler image, respectively.
 10. The ultrasonic imaging system ofclaim 1, wherein the beamformer further comprises a microbeamformercoupled to the array transducer and a main beamformer coupled to themicrobeamformer.